Multiple sclerosis (MS) is a nervous disorder which often strikes between the agesof 20 and 40 and is currently diagnosed in over 250,000 persons in the United States. Since the cause of MS is still unknown, drug therapies to date have been directed towards treating the symptoms of MS that range from loss of coordination, muscle weakness, slurred speech, and visual difficulties to paralysis. An important step forward in development of a drug to slow the progress of this disease, rather than merely treating the symptoms, has recently been made. In December, 1995, the FDA Scientific Advisory Panel voted unanimously for marketing approval of AVONEX* (interferon beta-1a), developed by Biogen, Inc. (Cambridge, MA) using genetic engineering technology. This drug is a recombinant human interferon beta product that is the biological equivalent to natural interferon beta that is produced by the body and plays and essential role in the immune system. Since the drug is only pharmacologically active in species closely related to humans, it was important to do safety testing in the nonhuman primate. Studies with a closely related form of this drug were carried out at the CRPRC in the rhesus monkey, which is a well-recognized reproductive and developmental model for humans. The effect of the drug on fertility and pregnancy was evaluated in order to determine safe levels of the compound for human use. Based, in part, on the results of these studies in the monkey model, this revolutionary drug is under final consideration for marketing approval for the treatment of patients with MS. This study serves as another example of the important contribution that nonhuman primate research has made to human medicine.